Most selenium is obtained commercially from various selenium-bearing materials which are by-products of other metallurgical or chemical processes. The materials are, for example, slimes, sludges, muds, dust and the like in which selenium is concentrated along with other valuable elements such as tellurium, silver, gold and platinum group metals. The method selected to extract the selenium from a material will depend on such factors as its composition, the form in which the selenium is present, availability of reagents, cost, and environmental considerations. Details on many processes now in use are given in books such as "Selenium", Ed. by R. A. Zingaro, pp. 31-60, (1974) and "Selenium and Selenides" by D. M. Chizhikov et al. pp. 57-115, (1968), "The Chemistry and Technology of Selenium and Tellurium" by A. A. Kudryavtsev, pp. 189-205, (1974) and in articles such as "Treatment of Electrolytic Copper Refining Slimes", by J. H. Schloen et al, J. of Metals, No. 5, pp. 764-777, (1950).
Since the most important source of selenium at the present time is anode slimes from electrolytic copper refining, the present invention will be discussed below with reference to the treatment of anode slimes. The principal methods of treating such slimes often include a preliminary decopperization step. One method for decopperization, for example, consists of treating the slimes with strong sulfuric acid at an elevated temperature. This method gives good copper extraction, decreases the total amount of material to be treated for selenium extraction, and increases the selenium content. The present method is particularly suited to slimes which have had an acid pretreatment. It is also particularly suitable for treating selenium-bearing material which also contains silver. Thus, the method of the present invention is discussed with particular attention to slimes which have been treated with sulfuric acid and which contain silver.
In one of the conventional routes for treating copper refinery slimes, the raw or decopperized slimes are mixed with soda ash and then roasted in an oxidizing atmosphere to convert selenium to water-soluble sodium selenite or selenate, which can then be leached. Selenium is recovered from the leach liquor by a number of known techniques. It has been recognized that this route requires a very intimate mixing of the selenium-containing particles with soda ash, a sufficient and continuous supply of oxygen reaching all the particles, and careful control of temperature so as to avoid a fusion which would prevent oxygen from penetrating the material. It is also necessary to provide sufficient soda to convert all the oxidized selenium to a non-valatile, water soluble selenite and/or selenate, so as to prevent selenium oxide volatilization.
Attempts have been made to provide the above requirements. U.S. Pat. Nos. 2,948,591 and 2,981,603, for example, disclose processes in which agglomerates are formed of the anode slimes and soda ash. However, the methods of these patents have shortcomings, particularly for anode slimes which have been decopperized by treatment in a strong solution of sulfuric acid. The processes of both patents involve forming mixtures of a pasty consistency out of which agglomerates are formed. This technique is suitable for processing dry materials and not applicable for treating wet selenium-bearing material containing sulfuric acid. Also the processes proposed by the two patents do not apply to materials in which the selenium is present mainly as both elemental selenium and silver selenide.
With respect to the slimes containing sulfuric acid, whatever the amount present, the H.sub.2 SO.sub.4 consumes the alkali carbonate, e.g. soda ash, which is added. Also, the soda interacts readily with many other sulfates, particularly sulfates of copper, nickel, iron, cobalt, lead, calcium, etc. All such reactions lead to conversion of the soda into sodium sulfate, which, in turn, is of no use for the formation of the water soluble selenium compounds during roasting. In addition, the actual amount of soda left to react with selenium oxides becomes unknown. Furthermore, it is impractical to mix the wet selenium-bearing slimes containing sulfuric acid with the alkali metal compounds needed for the roasting. Mixing Na.sub.2 CO.sub.3 and/or NaOH with the wet slimes containing H.sub.2 SO.sub.4 releases a large amount of heat due to neutralization of the H.sub.2 SO.sub.4 and, the mixture swells out because of CO.sub.2 evolution when Na.sub.2 CO.sub.3 is added, or it gets more moist with H.sub.2 O formed when NaOH is used. Also, the mixture sputters, contaminating the surroundings, as a consequence of either the CO.sub.2 evolution or steam generation. In either case, the heat of the sulfuric acid neutralization is capable of raising the temperature of the mixture up to the point where elemental selenium oxidation becomes possible, i.e. slightly above 200.degree. C. This highly exothermal oxidation to SeO.sub.2 tends to proceed so fast that the heat developed cannot be dissipated readily, and the temperature continues to rise. As a result it is not possible to achieve good mixing and there is a danger of SeO.sub.2 volatilization.
There is also the danger of SeO.sub.2 volatilization when the selenium is present mainly as elemental selenium and silver selenide. This is caused by the different conditions under which the elemental selenium and silver selenide react to form SeO.sub.2. It is well known that pure silver selenide is quite resistant to air oxidation up to 400.degree. C. while elemental selenium is oxidized and even can ignite slightly above 200.degree. C. There is no clue in either of the aforementioned patents on how to provide for completing all the chemical reactions involved while keeping the SeO.sub.2 from volatilizig.
Furthermore, while pure silver selenide is resistant to oxidation below 400.degree. C., at higher temperatures, especially at about 500.degree. C., it can readily form silver selenite, Ag.sub.2 SeO.sub.3, which is known to melt at about 530.degree. C. and decompose at 550.degree.-700.degree. C. When other compounds are present melting of silver selenite may occur at even lower temperatures, thereby obstructing oxidation. Another cause of disruption in oxygen supply may be the larger molar volume of sodium selenite and/or selenate formed as compared with sodium carbonate consumed. Both phenomena become real obstacles to complete oxidation when the selenium-bearing materials have Se and Ag contents somewhat above 10% each. They are not harmful when the materials contain about 8-10% or less of selenium and silver.